Electric Bacteria Could Be Used for Bio-Battery

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In an important step toward the creation of "bio-batteries," a
new study reveals how bacteria produce electricity when proteins
in their cell membranes come into contact with a mineral surface.

Scientists have known for some time that a family of marine
bacteria known as Shewanella oneidensis, found in deep ocean sediments
and soil, can create electrical currents when exposed to heavy
metals like iron and manganese.

In a study published today (March 25) in the journal Proceedings
of the National Academy of Sciences, researchers show that these
proteins can ferry electrons across a membrane at a rate fast
enough to produce the energy the bacteria need to survive.

Just as humans breathe oxygen and use it to generate energy,
Shewanella bacteria can use minerals like iron oxide for
respiration, study co-author Liang Shi, a microbiologist at
Pacific Northwest National Laboratory in Richland, Wash., told
LiveScience. The bacteria are known to produce a current by
shuttling electrons across their cell membranes, "but how this
electron transfer from bacteria to mineral occurs is not well
understood," Shi said.

There are two main possibilities for how it happens: The membrane
proteins might transfer electrons directly to the mineral
surface, or the proteins might use other molecules to help them
carry electrons across the cell membrane.

To show how membrane proteins in these
bacteria produce an electric current, researchers created a
bubblelike structure of fatty molecules studded with these
proteins, which mimicked the bacteria's cell membrane. It's much
easier to study these bubbles than real bacterial cells, which
are crowded with other structures, Shi said. The experiments were
also run in an oxygen-free environment, since oxygen can
interfere with the chemical reactions.

The bubbles contained an electron donor on the inside, and were
exposed to an iron-containing mineral on the outside. The
researchers measured the speed of the electrical current that
developed across the membrane.

The speed of this current was very fast — fast enough to suggest
the bacteria use this mechanism to create their electrical
currents in nature.

"Our research shows that these proteins can directly 'touch' the
mineral surface and produce an electric current, meaning that it
is possible for the bacteria to lie on the surface of a metal or
mineral and conduct electricity through their cell membranes,"
study leader Tom Clarke, a biologist at the University of East
Anglia, U.K., said in a statement.

Understanding how these bacteria function could enable scientists
to develop
bio-batteries that could store energy for sensors in remote
environments, for example. Conversely, the reverse process —
putting electricity into the bacteria — could be used to make the
bacteria manufacture useful materials.

Bio-batteries are already being developed, Shi said, though not
as part of this research. The next question is how these
electron-shuttling proteins fit into the whole system, not just
within the lab bubbles, he said.